Stuck fermentations drive winemakers crazy. Yeast that should be busy with the task of converting sugars into alcohol and carbon dioxide suddenly quit just short of the finish line, allowing bacteria strains to take over, consuming the sugars but not producing alcohol, only less desirable chemicals. It can spoil the whole tank, vat or barrel. After a year of hard work in the vineyards, a stuck fermentation can render the countless hours of labor and the beautiful fruit they produced useless.
But why do some vats get stuck? A team of researchers at the University of California at Davis have recently discovered a key piece to the puzzle. They hope it will usher in a new understanding of how fermentation works as well as a plan of attack to prevent it from getting stuck.
The key, according to Dr. Linda Bisson, a professor of viticulture and enology who focuses on yeast genetics, are prions. Pronounced PREE-ons, these are abnormal proteins scientists are just beginning to understand. These particular prions reside in the membrane of the yeast Saccharomyces cerevisiae, the de facto workhorse yeast of winemakers, brewers and bakers the world over.
Any vat of grape must is a competition for sugar between yeast and bacteria. Saccharomyces is very efficient at converting sugars into alcohol, so good that all that alcohol usually knocks out most strains of bacteria. But the UC Davis team observed that some bacteria strains can send some sort of chemical signal to the yeast cells, inducing them to produce a prion. That prion, they believe, triggers a condition in saccharomyces known as “glucose repression.” Their findings were published online in the journal Cell.
Normally saccharomyces is like the Cookie Monster, if it sees sugar, it eats sugar. Glucose repression tells the yeasts to try a more varied diet, consuming other sources of carbon too. That slows and effectively halts fermentation.
Stuck fermentation actually benefits both bacteria and the yeast, though not the winemaker. As alcohol production trails off, bacteria grow. The yeast benefit from a larger array of carbon to consume, allowing them to live longer, the team observed.
With this new understanding, the issue of stuck fermentation had to be looked at in an entirely new light. “What this does explain is why some vineyards are prone to sluggish fermentations even when nutrient levels [for yeast] seem reasonable and there is no apparent issue with temperature or alcohol tolerance," said Bisson. "The yeast are fermenting more slowly because of the induction of the prion and the fermentation rate will not respond to [adding] nutrients or air."
Bisson added that winery conditions don't appear to be a culprit, but vineyards can be. “We always knew we had these ‘difficult to ferment’ juices, but this work shows that [bacteria] can in fact induce the prion state, thus explaining the correlation between finding this bacterium and sluggish fermentations,” she said.
So what are the practical applications of this discovery for winemakers? “Winemakers may want to alter the levels of sulfur dioxide used when pressing or crushing the grapes, in order to knock out bacteria that can trigger the processes,” said Bisson. “They also can be careful about blending grapes from vineyards known to have certain bacterial strains. Of course the important thing is trying to find yeast that do not respond to the bacteria: So far the only ones that do not induce the prion are laboratory strains that cannot ferment well either, so we are actively screening wine yeast candidates.”